Boss has debts but is diligent (8)
I believe the answer is:
studious
'is diligent' is the definition.
(similar in meaning)
'boss has debts' is the wordplay.
'boss' becomes 'stud' (synonyms).
'has' means one lot of letters go next to another.
'debts' becomes 'IOUs'.
'stud'+'ious'='STUDIOUS'
'but' is the link.
(Other definitions for studious that I've seen before include "working hard" , "Deliberate and painstaking - preparing for exam?" , "Devoted to learning; deliberate" , "Like a good schoolboy" , "reading a lot" .)